Turbid media is media in which light scattering by constituent or generated irregular (randomly distributed) optical nonuniformities is of significant intensity. Scattering in turbid media leads to a change in the initial direction of the light irradiating the media. Examples of turbid media, but without limitation, include murky ocean water, atmospheric clouds, dust, sand, and biological tissue. While the present invention is directed toward an underwater scenario, the technique can be applied to other turbid media.
Dominance of the undersea environment (a turbid medium) is challenging due to the limitations of existing sensor technology. While radar technology is used extensively above the sea surface for communications, sensing, and navigation, the high absorption of radio frequencies by water prohibits the use of radar in the aquatic environment. Acoustic frequencies have been the preferred method for surveying the underwater environment due to low attenuation and long range propagation. However, acoustic techniques are limited in their ability to provide high resolution imagery for identification tasks, and acoustic frequencies cannot penetrate the air-sea interface. Laser-based sensors have been developed to fill in these performance gaps and have been integrated into both above-water and below-water platforms for underwater mine countermeasures. However, the size, weight, and power of these sensors are not compatible with small, unmanned and autonomous underwater vehicles that are being developed for undersea surveillance. This is primarily due to the fact that these existing systems incorporate transmitter and receiver hardware on the same platform.
To improve the compatibility of laser-based sensors with unmanned and autonomous subsea vehicles, researchers at the United States Navy have developed a technique where the transmitter and receiver are located on separate platforms (“Extended Range Optical Imaging System for Use in Turbid Media,” U.S. Pat. No. 8,373,862, issued. Feb. 12, 2013. This patent is hereby incorporated by reference, but not admitted to be prior art with respect to the present invention). This unique bistatic geometry enables the transmitter to optimize its distance from the object of interest so that the amount of light scattered on the path to the scene is minimized. As the source scans the underwater object, a time-varying intensity signal corresponding to reflectivity changes in the object is detected by the distant receiver. This time-varying intensity signal is not adversely affected by scattering on its path from the object. Since the laser illuminates only a small portion of the object of interest at a time, all the light that is reflected by the scene at each scan position carries useable information about the object. Thus, the receiver can collect all the light reflected by each pixel in the scene—even the light that is scattered multiple times on its path to the receiver—and still produce high quality images over large distances. To synchronize the laser and receiver, the laser is temporally encoded with information concerning the scan, such as scan rate, scan angles, etc., and the receiver decodes and uses this information to reconstruct the underwater image. The strength of this approach is that the transmitter and receiver are entirely autonomous and are linked only via a wireless communication signal that is carried by the light scattered from the object and from the environment Previous laboratory and in-situ experiments were conducted with the bistatic configuration and demonstrated the ability of the approach to collect high resolution imagery at up to 20 attenuation lengths between the receiver and underwater object. This previous approach was limited to collecting amplitude-only imagery. In order to measure range, the receiver must have a reference signal that is stable in time (phase locked) to the signal that is transmitted to and reflected from a scene of interest. The range is then measured by comparing the time delay of the scene-reflected signal to this reference signal. In the more conventional optical imaging approach where the transmitter and receiver are located on the same platform, this reference signal can be generated by sampling the transmitted signal (or the signal that modulates the laser). However, when the transmitter and receiver are on separate platforms with no connection between them, sharing a common reference becomes a challenge.